In retrofitting security systems of the type disclosed and claimed in the cross-referenced applications, the initial approach was to cut a slot in an existing symbol, such as a letter or number forming part of the trademark or model mark on the vehicle, and to glue a sensing coil in the slot. Although this retrofitting technique produced an operative system, it was not completely satisfactory for the reasons that the placement of the sensing coil in the symbol, which was formed of metal, degraded the merit factor or Q of the coil and also made it difficult to re-mount the symbol on the exterior of the automobile. In addition, there was little control over the positions from which the coil leads extended from the modified symbol with the coil inserted therein. The present invention involves the recognition of the aforementioned problems and disadvantages, and provides a solution therefor.